What was the cause of the damage? I have never seen heads freeze and not the block. I have seen both heads crack from over heating or running extremely lean in a demolition derby though.
Anyhow, you should be able to go to almost any engine machine shop and get a set of reconditioned heads for about $250-400 complete. Just male sure they match each other in port and valve size and that a good valve job was done during the rebuild. You can get brand new, complete performance heads from GM Performance 25534421 for about $400 if you shop around, list is $529.99
Anyhow, if you are even remotely skilled, you should be able to do the heads in a weekend. You can even do it in the boat..